The Alt+Tab key sequence on Windows is the common power-user method of
switching between application windows. I use this frequently, especially when
running with an IDE like NetBeans; you run the application from within the IDE
and Alt+tab back and forth to check what the app is doing compared to what the
code is telling it to do.
Alt+Tab basically works on Netbeans, but typically the "Alt" key modifier is
left active in Netbeans. This means when you come back to NetBeans and try to
do anything (like type some code) the IDE will interpret the keys you enter as
Alt commands. (You can see that the Alt key is still active when the "File"
menu is selected, even though you have not selected it; hitting "Alt" alone in
the IDE will do this same selection).
This is at the very least an annoyance, as the keys you enter go largely
ignored until you realize the problem and hit Esc or click in the window to
make the Alt state go away. But depending on what keys you type, you may get
some very undesirable result when some Alt command occurs that you did not
expect or want.
Also, this is very non-native behavior; no other Windows native app has the
problem of continuing to track the Alt key state even when that app is
inactive.

Unfortunately this is known JDK bug, nothing we can do on Netbeans side.
See http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6458497
Appearance of this bug relates to the time and the way how Alt+Tab is pressed.
When you press it slowly and first the Alt, then the Tab, release Tab, release
Alt, error is more reproductible.